When oil wells are drilled in the earth, it is common practice to set a string of steel in the well bore. The casing is perforated at the levels or intervals at which it is desired to extract oil from the surrounding formation, and these perforations require periodic cleaning to ensure unobstructed flow of oil from the group and into the casing. The present invention relates to a tool for washing perforations in a cased bore hole, and more particularly, to a tool for opening or cleaning the perforations in the casing for permitting unobstructed injection of material or entry of well fluids into the well casing.
At the present time, there are a number of tools available for washing the perforations of a well bore casing or for injecting into perforations. One such tool is a selective injection tool which operates on the principle of injecting fluid into each perforation, by pumping the washing fluid down the tubing and returning the fluid to the surface through the annulus between the washing tool and the casing of the bore. This type of tool requires a high pumping rate in order to carry the debris or sand in water back to the surface. A method of washing commonly employed by this type of tool comprises injecting five barrels of wash water, and then reverse circulating this water to the surface of the well. This procedure is obviously time-consuming, because the entire capacity of the tubing must be circulated for each perforation.
There are other known wash tools similar to that described above, but having an additional cup positioned above the cups which surround the perforations. After injecting into each perforation by pumping down the tubing, the tool is raised to a position where the additional cup is just below the uppermost perforation. Fluid is then pumped down the annulus at a very high rate, with the object of being diverted into the top perforations. These tools accomplish very little washing, however, because the tools are provided with an internal by-pass which permits most of the fluid to be exhausted out of the lower end of the tool, rather than through the perforations.
Another washing tool is disclosed in U.S. Pat. No. 3,760,878 issued to Peevey on Sept. 25, 1973. This patent describes a tool which permits washing of each perforation by pumping a fluid down the annulus and returning the debris or sand to the surface by a path up the tubing. This device has the disadvantage that the entire casing string must be pressurized in order to inject into a perforation. If the perforation to be washed has a high initial injection pressure, the high washing pressure may exceed the safe recommended pressure for the well casing.
The present invention overcomes each of the drawbacks noted above, and provides a washing tool which can safely and efficiently perform washing operations on each casing perforation regardless of the initial breakdown pressure, and which is also capableof reverse circulation of debris to the well surface.